1. Field of the Invention
The present invention relates to a technology for correcting positional misalignment between images in different colors in an image forming apparatus.
2. Description of the Related Art
In tandem-type image forming apparatuses, such as color copiers and color laser printers, image forming processing is performed such that toner images are formed using toners that are developing agents in four colors of yellow, cyan, magenta, and black, and the toner images are sequentially superimposed one on top of the other onto a transfer member (a transfer belt or a transfer paper). Because the toner images are sequentially superimposed, relative positions of the toner images may be misaligned, which leads to color shift. The color shift significantly degrades quality of a color image formed by superimposing the toner images onto the transfer paper. Therefore, it is necessary to suppress color shift (positional misalignment) in the image forming apparatuses.
For example, Japanese Patent Application Laid-open No. 2005-31227 discloses a conventional positional misalignment correcting device that corrects positional misalignment by optically reading a positional misalignment correction pattern formed of a plurality of patches. The positional misalignment correction pattern is formed on an intermediate transfer member such that a reference color pattern and a target color pattern to be corrected (correction toner image) are overlapped with each other. The positional misalignment correcting device includes a detecting unit and a correcting unit. The detecting unit detects specular reflection components, diffused reflection components, or both when a reflective photosensor optically reads the positional misalignment correction pattern. The correcting unit corrects the positional misalignment based on the detected specular reflection components, diffused reflection components, or both. The positional misalignment correcting device sets gloss level of the intermediate transfer member based on an output of the specular reflection components and sets luminosity based on an output of the diffused reflection components outputted when the reflective photosensor optically reads the positional misalignment correction pattern.
Furthermore, Japanese Patent Application Laid-open No. 2002-236402 discloses an image forming method and an image forming apparatus in which a color toner reference image (correction toner image) is formed on an image carrier or a transfer member carrier. A diffused reflection-type concentration detecting unit and a specular reflection-type concentration detecting unit detect reflected light from the reference image. An output value from the diffused reflection-type concentration detecting unit is corrected based on an output value from the specular reflection-type concentration detecting unit and the output value from the diffused reflection-type concentration detecting unit at the time of detection.
In the conventional technologies as described above, the correction toner image is detected by a detector including two light-receiving elements for receiving the specular reflection components and for receiving the diffused reflection components while including a single light-emitting element. When a detector is provided with only one light-receiving element, size and cost of the detector can be reduced as a result of the correction toner image being detected based only on the specular reflection components received by the single light-receiving element.
When the detector is disposed such that an optical axis of the light-emitting element and an optical axis of the light-receiving element on a plane parallel to a normal line direction of the transfer member intersect on a front surface of the transfer member, and an angle formed by the optical axis of the light-emitting element and a normal line of the transfer member and an angle formed by the optical axis of the light-receiving element and the normal line match, a large portion of the reflected light received by the light-receiving element is the specular reflection components. Therefore, effects of the diffused reflection components can be substantially ignored. However, when the optical axis of the light-emitting element and the optical axis of the light-receiving element become misaligned as a result of manufacturing variations in the detector and the like, the effects of the diffused reflection components within the reflected light received by the light-receiving element cannot be ignored. Therefore, detection precision of the detector may decrease.